This invention concerns a refrigeration unit comprising compartments at different temperatures.
Such units usually take the form of a refrigerator, a deep-freeze, a refrigerated cabinet, or a cold-storage chamber. The commonest form is a refrigerator with two compartments, each kept at a relatively constant temperature by two evaporators with different temperature levels. There is a refrigerator compartment, kept at approximately +5.degree. C., and a freezer compartment, kept at approximately -18.degree. C.
Most existing refrigerators of this type operate on a single compressor, which cools the two evaporators, connected in series. Under this method, distribution of cold between the two compartments is governed by the construction of the refrigerator, and cannot be adapted to needs, which may vary, depending partly on use (opening of doors, insertion of products for cooling or freezing) and partly on atmospheric temperature. When this falls, not enough cold is produced in the freezer, so that the necessary -18.degree. C. is not attained. This can be remedied by heating the refrigerator compartment with a resistance, or by ensuring that the compartments are properly balanced at temperatures of -18.degree. C. and +5.degree. C. for a given lowest outside temperature, for example +16.degree. C.; in this case, however, an outside temperature of +32.degree. C., for example, will result in temperatures of -23.degree. C. and +5.degree. C. Either solution, resistance or colder thermal balance, also means extra energy consumption; in addition, they function satisfactorily only within a restricted temperature range, and for a limited freezer capacity (approximately 25% of the total volume of the unit, with an absolute limit of about 100 liters).
Both compartments can also be cooled by evaporating the cold-generating fluid at the temperature needed for the colder compartment, but this means inefficient energy consumption.
Other refrigerators are equipped with two compressors, each supplying cold to one compartment. This ensures the right temperatures on both compartments, regardless of their volume ratio to each other, and allows them to be regulated separately. But it is much more expensive: such refrigerators consume at least as much energy as those mentioned earlier, since the fixed wastages from each compressor, although individually lower, are combined.
Many methods have been suggested for allowing two compartments to function independently, in a refrigerator equipped with only one compressor. Most of them involve an electro-valve, which halts one evaporator, usually the one cooling the refrigerator compartment, while the other continues to function. But this does not really provide the fully independent temperature regulation and stability because the cold circuit functions quite differently, depending on whether only one or both evaporators are being cooled, and in order to obtain reasonably adequate results, more sophisticated and consequently much more expensive regulation devices are needed.
Other suggestions involve a secondary circuit to cool the refrigerator compartment. This circuit is in thermal contact with the freezer compartment, and heat exchange in the circuit is controlled by an electro-valve or throttle, which can be heated to halt fluid circulation.
This solution does not provide the same level of independence between the two compartments as the use of two separate compressors and circuits, which allow temperatures to be regulated quite separately, with the freezer compartment kept functioning permanently, for deep-frozen products and to improve freezing.
In addition, the evaporation temperature of a single-compressor appliance is dictated by the requirements of the colder compartment, detracting from thermodynamic efficiency.
The drawbacks mentioned above are particularly noticeable in the case of refrigerating circuits in which a capillary tube is used to reduce the pressure of the refrigerant from a high level at the condenser outlet to a low level at the evaporator inlet or inlets; this is because the flow in a capillary tube is not stopped but varies to suit conditions prevailing on the high-pressure and low-pressure sides. The capillary tube possesses a self-regulating effect, which allows it to adapt to various conditions occurring during actual operation. This capacity, combined with its simplicity and reliability, has resulted in its widespread use for domestic refrigeration.
This invention provides a way of overcoming the drawbacks described, by proposing a refrigeration unit having compartments at different temperatures, separately regulated. In this unit, supplies of cold for each compartment are made independent of each other by means of a thermal valve, and temperature regulation is kept independent by alternate functioning; cold is generated for only one compartment at a time, allowing the temperature of each compartment to be kept more steady.
In a capillary system, such a result can be obtained only with the device described in this invention, using a system comprising a secondary circuit to cool the freezer compartment, acting as a thermal valve and comprising one section acting as evaporator and situated inside the freezer, and another section acting as condensor, and forming a heat exchanger with one of the primary circuit evaporators.
Expansion valves, controlled by the evaporation temperature or by other means, allow several compartments to be kept independent of one another more easily. These valves can be used to halt the flow of fluid, and thereby make the circuit function with a reserve of liquid, ensuring far greater independence of the compartments, each of which is regulated by a thermostatic valve. This method is used in commercial refrigeration. However, these expansion valves are far too expensive to be used in domestic refrigeration.